Electronic Properties of 4,4′,5,5′-Tetramethyl-2,2′-biphosphinine (tmbp) in the Redox Series fac-[Mn(Br)(CO) 3 (tmbp)], [Mn(CO) 3 (tmbp)] 2 , and [Mn(CO) 3 (tmbp)] - : Crystallographic, Spectroelectrochemical, and DFT Computational Study Frantis ˇek Hartl* and Taasje Mahabiersing Institute of Molecular Chemistry, UniVersiteit Van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Pascal Le Floch,* Francois Mathey, Louis Ricard, and Patrick Rosa Laboratoire “He ´ te ´ roe ´ le ´ ments et Coordination”, URA CNRS 1499, DCPH, Ecole Polytechnique, 91128 Palaiseau Cedex, France Stanislav Za ´lis ˇ J. HeyroVsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ˇkoVa 3, 182 23 Prague 8, Czech Republic Received November 25, 2002 Stepwise electrochemical reduction of the complex fac-[Mn(Br)(CO) 3 (tmbp)] (tmbp ) 4,4′,5,5′-tetramethyl-2,2′- biphosphinine) produces the dimer [Mn(CO) 3 (tmbp)] 2 and the five-coordinate anion [Mn(CO) 3 (tmbp)] - . All three members of the redox series have been characterized by single-crystal X-ray diffraction. The crystallographic data provide valuable insight into the localization of the added electrons on the (carbonyl)manganese and tmbp centers. In particular, the formulation of the two-electron-reduced anion as [Mn 0 (CO) 3 (tmbp - )] - also agrees with the analysis of its IR ν(CO) wavenumbers and with the results of density functional theoretical (DFT) MO calculations on this compound. The strongly delocalized π-bonding in the anion stabilizes its five-coordinate geometry and results in the appearance of several mixed Mn-to-tmbp charge-transfer/IL(tmbp) transitions in the near-UV-vis spectral region. A thorough voltammetric and UV-vis/IR spectroelectrochemical study of the reduction path provided evidence for a direct formation of [Mn(CO) 3 (tmbp)] - via a two-electron ECE mechanism involving the [Mn(CO) 3 (tmbp)] • radical transient. At ambient temperature [Mn(CO) 3 (tmbp)] - reacts rapidly with nonreduced fac-[Mn(Br)(CO) 3 (tmbp)] to produce [Mn(CO) 3 (tmbp)] 2 . Comparison with the analogous 2,2′-bipyridine complexes has revealed striking similarity in the bonding properties and reactivity, despite the stronger π-acceptor character of the tmbp ligand. Introduction Since the mid-1970s the coordination chemistry of Re(I) carbonyls with chelated R-diimine ligands such as 2,2′- bipyridine (bpy), [Re(L) m (CO) 4-m (R-diimine)] n (m ) 0-2; n ) 0, +1; L ) e.g. donor solvent, halide, phosphine), has received wide interest. Many of these complexes have been extensively investigated owing to their electrocatalytic properties in CO 2 reduction 1 and ability to participate in efficient inter- and intramolecular energy- and electron-trans- fer processes. 2 By contrast, only a few analogous Mn(I) compounds [Mn(L) m (CO) 4-m (R-diimine)] n (m ) 1-3; n ) 0, +1) have been prepared and studied so far, even though * Authors to whom correspondence should be addressed. E-mail: hartl@science.uva.nl (F.H.); lefloch@poly.polytechnique.fr (P.L.F.). (1) See for example: (a) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J. Organometallics 1996, 15, 3374-3387. (b) Christensen, P. A.; Hamnett, A.; Muir, A. V. G.; Timney, J. A. J. Chem. Soc., Dalton Trans. 1992, 1455-1463. (c) Calzaferri, G.; Hadener, K.; Li, J. W. J. Photochem. Photobiol., A 1992, 64, 259-262. (d) Koike, K.; Hori, H.; Ishizuka, M.; Westwell, J. R.; Takeuchi, K.; Ibusuki, T.; Enjouji, K.; Konno, H.; Sakamoto, K.; Ishitani, O. Organometallics 1997, 16, 5724-5729. Inorg. Chem. 2003, 42, 4442-4455 4442 Inorganic Chemistry, Vol. 42, No. 14, 2003 10.1021/ic0206894 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/12/2003